I believe the story would not be as effective if the reader knew at the onset that the story happened fifty years ago. When reading the story, it appears to be happeneing now. It is quite effective in that the reader thinks that Fortunato is receiving his punishment for the thousands of injuries he has done to Montresor.
If the reader had known that the whole story happened fifty years ago, it would not have had the same effect. It would not have been as intriguing and mysterious. Knowing that the story happened fifty years ago loses the effect of the present injury.
Fifty years is a long time. To think that the story happened fifty years ago loses the effect of justice that seems to be occuring in the very present tense.
There is an intensity that occurs because the story seems to happening in the present tense. I am glad Poe saved that information for the last. Even reading it at the end, it changes the intensity of the story.
Who cares what happened fifty years ago? There is a definite change in the overall mood knowing that it happened fifty years ago. Who can remember yesterday? Fifty years ago is a bit of a stretch?
I would dare say that the immense struggle that is occurring in the story is effective because the reader is thinking it is happening in the present tense. Afterall, that is what makes a classic a classic. Its enduring presence has stood the test of time:
A classic, through its enduring presence, has withstood the test of time and is not bound by time, place, or customs. It speaks to us today as forcefully as it spoke to people one hundred or more years ago, and as forcefully as it will speak to people of future generations. For this reason, a classic is said to have universality.
Thinking that the story happened fifty years ago does change the intensity of Montresor's purpose.
When the reader is unsuspecting of Montresor's plan, there is an element of surprise as the reader discovers what Montresor is planning to do to Fortunato. The element of suspense keeps the reader interested:
Fortunato does not suspect Montresor's plan. In fact, when they meet in the street during carnival, Fortunato is very glad to see him.
The element of surprise is what intrigues the reader. Knowing that the story has happened fifty years ago is a definite let down.